Reduced adverse effects of infrared free electron laser-irradiated insulin amyloid in vitro and in mice.

Insulin is a hormone that physiologically contributes to the control of glucose metabolism and therefore insulin self-injection has been one of the highly effective therapeutic approaches for diabetes. However, repeated injection of insulin preparations into the same site induces the formation of insulin amyloid. The amyloid adheres to native insulin, reduces the absorption of native insulin into blood vessels and eventually leads to high blood glucose levels. However, less-invasive radical treatments have not been explored. In this study, we examined whether free electron laser (FEL) irradiation of insulin amyloid reduces the toxicity elicited by the amyloid. FEL irradiation decreased the size of insulin amyloid as proved by transmission electron microscopy (TEM) and circular dichroism (CD). In addition, amount of the insulin with a beta-sheet structure was reduced after FEL irradiation, as evidenced by thioflavin T (ThT) assay. An ELISA experiment revealed reduced adhesion of FEL-irradiated insulin amyloid to native insulin. In mice, FEL-irradiated insulin amyloid formed sparse nodules in subcutaneous tissues compared to those formed by non-irradiated amyloid. These results suggest a potential for application of FEL to insulin amyloid, provided that direct irradiation of FEL to insulin amyloid in mouse skin leads to improved blood glucose levels in future.